1. Field of Invention
This invention is directed to a process for increasing the stick temperature of ethylene-based polymer pellets and to the resulting pellets that have resistance to blocking under storage conditions and under conventional operating temperatures used to process the polymer pellets.
2. Related Art
Polymer pellets after their manufacture as well as during shipment and subsequent handling, such as when the pellets are charged into an extruder for further processing to form articles, need to remain free flowing and not stick together or agglomerate at conventional storage, shipping and operating temperatures. Additives that reduce blocking or sticking can be melt blended into the polymer before it is pelletized or an additive can be applied to the surface of the pellet. In either case, the additive should remain effective from the time the pellet is formed to when the customer uses the pellet. The additive must not cause operability problems in equipment during handling either while the pellet is being formed, conveyed, packaged or in the customer""s plant. The additive must not negatively affect the end use properties of the product and must meet regulations pertaining to food contact in the event the product is used, for example, for food packaging.
Donaldson et al., U.S. Pat. No. 3,528,841, issued Sep. 15, 1970, teaches the use of a copolymer of ethylene oxide and propylene oxide on polymeric pellets to reduce tackiness. Pugh et al., U.S. Pat. No. 3,969,304, issued Jul. 13, 1976, shows the use of slip agents for polyethylene films of various inorganic powders and organic materials such as erucamides, oleamides, sulfonates, stearates, stearamides, and the like. Kelly et al., U.S. Pat. No. 4,769,289, issued Sep. 6, 1988, teaches the use of fumed silica on polar ethylene interpolymers to form free flowing polymer particles. Smith, U.S. Pat. No. 5,286,771, issued Feb. 15, 1994, shows various alcohols, ethers, alkylene oxide polymers, and oils as additives to reduce blocking of polymer pellets. Japanese Patent Kokai No. Sho 60(1985)-23449, published Feb. 2, 1985, teaches the use of an alkali metal salt of a higher aliphatic acid having 8-22 carbon atoms or an ester of such an acid to reduce the adhesion problems of thermoplastic resin particles or pellets. However, none of these patents are concerned with reducing the blocking of pellets of ethylene-based copolymers having a relatively high acid content, for example, polymers having an acid content of 5-30% by weight.
While co-pending application U.S. Ser. No. 09/749,093, which is incorporated herein by reference, does address the problem of blocking of ethylene-based copolymers having relatively high acid content, it presents other problems, particularly handling difficulties. The invention in this co-pending application requires application of a metal salt of a C12-22 aliphatic acid to the ethylene-based polymers. These metal salts are solids that must be applied either as a dry powder or from aqueous dispersions, which typically also contain surfactants to disperse the solid metal salts in the aqueous medium. Because these metal salts are solids, they present problems normally associated with solids handling, such as settling in pipe lines and vessels used for making or storing slurries, erosion, dust creation, and added energy and maintenance costs associated with need for agitation to keep solids suspended.
The process of the present invention solves these handling problems while still raising stick temperature of pellets sufficiently to avoid blocking problems.
This invention is directed to a means for increasing the stick temperature (particularly to at least 25xc2x0 C., particularly at least 5xc2x0 C. above the stick temperature of the untreated pellet) of ethylene-based acid copolymer pellets having an acid content at least 5% by weight, particularly copolymers having acid content of about 5 to about 30% by weight. A solution of one or more water-soluble metal salts of aliphatic acids having fewer than twelve carbons, preferably soluble metal salts of C4-10 aliphatic acids, is applied to the pellets, and the pellets are dried. The metal salts are preferably in an aqueous solution in an amount sufficient to increase the stick temperature to above 25xc2x0 C., preferably by 5xc2x0 C. or more above that of the pellets before treatment.
The dried pellets have an effective amount of a surface coating of the metal salt of the C4-10 aliphatic acid to increase the stick temperature of the copolymer to a temperature above 25xc2x0 C. Preferably, the amount of coating is sufficient to raise the stick temperature at least 5xc2x0 C. above that of the untreated pellet. Preferred ethylene-based copolymer pellets comprise repeating polymerized units of
(a) at least 50% by weight, based on the weight of the copolymer, of ethylene,
(b) 5 to 30% by weight, based on the weight of the copolymer, of acrylic acid or methacrylic acid; and
(c) 0 to 40% by weight, based on the weight of the copolymer, of an alkyl acrylate or an alkyl methacrylate,
wherein the copolymer has 0-100% of the acid groups neutralized with metal cations, particularly those selected from the following group: zinc, magnesium, sodium and lithium, and the surface coating comprises an effective amount of one or more metal salts (preferably a sodium salt) of one or more aliphatic acids having fewer than 12 carbon atoms, preferably water-soluble metal salts of aliphatic acids having 4 to 10 carbon atoms, generally in an amount of 25 to 6,000 ppm (parts by weight metal salt of aliphatic acid per million parts by weight of copolymer).